Polyetherimides for Improved Processability of PIs
Polyetherimides for Improved Processability of PIs
Polyimides are a relatively new class of specialty plastic materials that are characterized by:
- High strength-to-weight ratio
- Thermo-oxidative stability
- Excellent mechanical properties
- High temperatures resistance and more…
The characteristic group of polyimides is the imide or –C=ONC=O- group.
Polyetherimides have been developed to overcome challenges associated with polyimides i.e. this polymer family is not readily melt processable, and finished parts tend to be rather expensive.
Incorporating the proper ether linkages into the polyimide molecular chain provides sufficient flexibility to allow good melt processibility yet retains aromatic imide characteristics of excellent mechanical and thermal properties.
- Imides impart high temperature performance
- Ether groups allow melt processing
Polyetherimide was first developed in 1982 by General Electric Company (now known as SABIC) under the trade name ULTEM™ resin.
Molecular Structure of Polyetherimide (PEI)
Today, PEI is available from several suppliers such as: SABIC, RTP Company, Lehmann & Voss, Quadrant, PolyOne etc.
Polyetherimide (PEI) is an amorphous engineering thermoplastic known to exhibit high temperature resistance, outstanding mechanical and electrical properties.
- Molecular Formula: [C37H24O6N2]
- Molecular weight: 592.61 g/g-mol
This high-performance polymer also exhibits high tensile strength, good flame resistance and low smoke emission making it an ideal material of choice in automotive, electrical, medical and other industrial applications.
The high-temperature resistance of Polyetherimide competes with polyketones, polysulfones, and polyphenylene sulfide.
Synthesis of Polyetherimide
Synthesis of Polyetherimide
PEI is produced via the polycondensation reaction between bisphenol-A dianhydride such as tetracarboxylic dianhydride (produced from the reaction of bisphenol A and phthalic anhydride) and a diamine such as m-phenylene diamine.
The early laboratory process involved a costly and difficult synthesis. Further development resulted in a number of breakthroughs that led to a simplified, cost-effective production process. The final step of the process involves the imidization of a diacid anhydride with m-phenylene diamine as shown in Figure above. PEI has a Tg of 217 °C.
Key Properties of PEI
Key Properties of PEI
The basic characteristics and key features of Polyetherimide are already discussed. Apart from them, there are various other properties that need to be considered before choosing a particular thermoplastic for a required end-use application.
- PEI is an amorphous thermoplastic resin with amber transparency
- The resin is characterized by high deflection temperature (200°C at 264 psi), high tensile strength and flexural modulus (480,000 psi), and very good retention of mechanical properties at elevated temperatures
- It has unique combination of high specific strength, rigidity, flexibility, exceptional dimensional strength etc.
- In addition, the resin exhibits good electrical properties, which remain stable over a wide range of temperature and frequencies (including microwave)
- It has good UV-light resistance and weatherability
- PEI is inherently flame resistance without the use of additives
- It has a high limiting oxygen index of 47, combined with NBS smoke chamber results which show the lowest specific optical density of any unfilled thermoplastic
- Polyetherimide is resistant to alcohols, acids, and hydrocarbon solvent but dissolves in partially halogenated solvents
- PEI also displays good hydrolytic stability
- Most of the PEI grades has a UL94 flame resistance rating of VTM-0, is FDA compliant, EU Food Contact Compliant, and ISO10993 compliant in natural color
Polyetherimide resin is available in an unreinforced grade for general-purpose processing methods as a transparent resin and in standard and custom colors.
It is also available in:
Glass reinforcement provides even greater rigidity and dimensional stability while maintaining many of the useful characteristics of basic PEI. The glass reinforcement yields a product with an exception strength-to-weight ratio and increased tensile strength.
Polyetherimide (PEI) Performance Versus Sulfone Polymers
Source: SABIC
Limitations Associated with PEI
- Very high cost - applicable for highly demanding applications
- Low colorability
- Attacked by polar chlorinated solvents, aromatic hydrocarbons, acetates, etc. leading to stress cracking (See chemical resistance performance of PEI with sulfone polymer in the table below)
- A long drying before processing is necessary
- Hot mold during injection molding
| Chemical |
ULTEM™ |
PES |
PSU |
PPSU |
| Aromatic Hydrocarbons |
3 |
1 |
1 |
4 |
| Aliphatic Hydrocarbons |
10 |
10 |
7 |
10 |
| Chlorinated Hydrocarbons |
3 |
3 |
1 |
5 |
| Alcohols |
10 |
7 |
7 |
10 |
| Inorganic Base |
1 |
10 |
10 |
10 |
| Acetates / Ketones |
4 |
1 |
1 |
4 |
| Acids |
7 |
7 |
7 |
7 |
How to Process PEI?
How to Process PEI?
Unlike most other polyimides, PEI is suitable for processing by typical methods such as:
- Injection Molding
- Extrusion
- Thermoforming
- Compression molding
Polyetherimides can be melt processed because of the ether linkages present in the backbone of the polymer. However, it still maintains high-temperature properties similar to the polyimides. The polymer should be dried at 140-150°C for 4 to 6 hours before processing for a maximum moisture content of 0.01% to 0.02%.
- Processing temperature: 370 to 400°C
- Purging of the barrel is necessary when changing material (PC or HDPE at 370-400°)
- Mold Temperature 65 to 180°C
- Suitable for injection of very small parts with tight dimensional tolerances
The polymer can be easily machined with conventional metalworking tools, painted, hot stamped, printed, or metallized.
PEI films are made via melt-extrusion and solvent casting processes and PEI fibers are made via melt-extrusion and spinning.
Bonding of PEI is carried out typically via:
- Fuse bonding
- Ultrasonic bonding
- Solvent bonding
- Adhesive bonding
Additionally, PEI is nowadays widely used resin material to create functional prototypes and production parts for high-strength, FST-rated and certified applications using 3D Printing.
Key Properties
Key Properties
|
| Property |
UNFILLED PEI |
30% GFR PEI |
| Electrical |
| Arc Resistance, sec |
128 |
85 |
| Dielectric Constant |
3.1 - 3.2 |
3 - 4 |
| Dielectric Strength, kV/mm |
28 - 33 |
25 - 30 |
| Dissipation Factor x 10-4 |
13 - 25 |
15 - 53 |
| Volume Resistivity x 1015, Ohm.cm |
5 - 18 |
15 - 16 |
| Mechanical |
| Elongation at Break, % |
59 - 60 |
3 |
| Elongation at Yield, % |
6.8 - 7.2 |
3 |
| Flexural Modulus, Gpa |
3 - 3.4 |
9 |
| Hardness Rockwell M |
100 - 110 |
90 - 125 |
| Hardness Shore D |
95 - 99 |
95 - 99 |
| Strength at Break (Tensile), MPa |
90 - 100 |
150 - 160 |
| Strength at Yield (Tensile), MPa |
100 - 110 |
150 - 160 |
| Toughness at Low Temperature (Notched Izod Impact at Low Temperature), J/m |
50 - 60 |
90 - 100 |
| Young's Modulus, GPa |
3 |
9 |
| Physical |
| Density, g/cm3 |
1.27 - 1.3 |
1.5 - 1.6 |
| Gamma Radiation Resistance |
Good |
Good |
| Glass Transition Temperature, °C |
215 |
215 |
| Shrinkage, % |
0.7 - 0.8 |
0.2 - 0.4 |
| Sterilization Resistance (Repeated) |
Good |
Good |
| UV Light Resistance |
Fair |
Fair |
| Water Absorption 24 hours, % |
0.2 - 0.3 |
0.1 - 0.2 |
| Service Temperature |
| HDT @0.46 Mpa (67 psi), °C |
195 - 210 |
205 - 212 |
| HDT @1.8 Mpa (264 psi), °C |
190 - 200 |
200 - 210 |
| Min Continuous Service Temperature, °C |
170 |
170 |
| Thermal |
| Coefficient of Linear Thermal Expansion x 10-5, /°C |
5 - 6 |
2 |
| Fire Resistance (LOI), % |
46 - 47 |
50 |
| Flammability, UL94 |
V0 |
V0 |
| Thermal Insulation, W/m.K |
0.22 - 0.25 |
0.23 - 0.26 |